1. Field of the Invention
This invention relates to a method and an apparatus for overwritable magneto-optical recording.
2. Description of the Prior Art
An optical disk allows higher-density recording than a magnetic disk, but is inferior in high-speed performance, with an access time that is several to ten times as long. A method for remedying this is overwriting. Conventionally, rewriting is done during three turns: erasing on the first turn, recording on the second, and error checking on the third. In contrast, overwriting will allow rewriting during only two turns, by erasing and recording simultaneously. One approach that uses two heads for erasing and recording to permit pseudo-overwriting has been proposed for overwritable magneto-optical recording. However, the system is complicated. Another method that allows simultaneous erasing and recording of adjacent tracks with two laser spots cannot be used in sector-based writing. On the other hand, a field modulation method, a light modulation method using an exchange-coupled double-layered film, and a light modulation method using a demagnetizing field have been proposed as direct overwrite methods. Of these, the light modulation overwrite method using an exchange-coupled double-layered film is superior to the field modulation system in that it provides easy high-speed modulation and allows a wide space between a bias magnet and a disk. However, there are problems in that the properties of the media must be controlled accurately, and that the laser power margins for reading and writing become smaller. The double-layered film overwrite method disclosed in JA Published Unexamined Patent Application (PUPA) No. 62-175948 is explained below.
The recording layer used in this method consists of two layers, a memory layer and a reference layer, which are exchange-coupled. Overwriting is performed by utilizing the difference in temperature dependence of the coercive forces (Hc) of the two layers. FIG. 15 shows the magnetic properties of an overwritable disk, and FIG. 16 shows a method of overwriting on this type of medium. Basically, this method utilizes the magnetic properties that the coercive force of the reference layer (Hca2) is smaller than that of the memory layer (Hca1) at room temperature (Tamb1) and that the Curie temperature of the reference layer (Tc2) is higher than that of the memory layer (Tc1). The magnitude of the external field Hini applied before laser emission is set so that Hca2&lt;Hini&lt;Hca1. First, only the magnetization of the reference layer follows Hini. In the example shown in FIG. 16, the direction of the magnetization of the reference layer becomes downward. The memory layer retains the recorded information, because Hca1 is large.
When a recording medium in this state is irradiated with a low-power laser beam, the temperature of the recording layer TL becomes Tc1&lt;TL&lt;Tc2. Thus, the magnetization of the reference layer is not reversed by the weak external field Hb, so that the magnetization of the memory layer is oriented downward during the cooling process, following the magnetization of the reference layer, as shown in FIG. 16 (erasing). Thus, although the directions of the magnetic moments of similar atoms become essentially parallel to each other, in this case the directions of magnetization also become parallel to each other. When the laser power is high, the temperature of the recording layer TH becomes higher than Tc2. Thus the magnetization of the reference layer is reversed by the external field Hb, so that during the cooling process (writing) the magnetization of the memory layer is oriented in the same direction, that is, upward in FIG. 16. Recording by using the magnetization of the reference layer is hereinafter referred to as the "L process" and recording by using an external field is referred to as the "H process." The name of each process is derived from the temperature of the reference layer.
FIG. 17 shows the experimental results of the overwriting characteristics for a double-layered film whose memory layer consists of TbFeCo and whose reference layer consists of DyFeCo, reported by Matsumoto Hiroyuki, et al. in "Magneto-optical Disk for Direct Overwrite by Light Power Modulation Method," Symposium on Optical Memory '88, Article Collection 45 (1988). The right side of FIG. 17 shows the carrier level in the H process, which is a recording process with high laser power. The results are obtained by using a modulated light of 1 MHz to record on the medium with its data erased. The left side of FIG. 17 shows the signal level of data left unerased during the L process, which is an erasing process with low laser power. The results are obtained by using CW (constant wavelength) unmodulated light to erase signals of 1 MHz written onto a medium.
When the H/L processes are done by modulating the power of pulses of long duration (the duration is hereinafter referred to as "pulse width") of the order of one hundred nanoseconds conventionally, it is necessary to divide the laser power level into three regions: PR (reading), PL (L process), and PH (H process). Accordingly, the margin for each of them is lower than when overwriting is not performed, and therefore it is sufficient to provide only two power levels for reading and writing. Furthermore, since Tc2 is set at a high temperature to obtain a margin for laser power in the L process (PL), the medium is exposed to a high temperature in the H process, which may cause deterioration. Other problems are that the decreased margin requires accurate control of the properties of the medium, and that the characteristics of write/read are apt to be influenced by fluctuations in the laser power and ambient temperature.
The magnetic recording medium disclosed in JA PUPA No. 62-80847 comprises a memory layer for retaining domains with desired field orientations representative of binary data, a reference layer for providing a bias field as a function of the temperature for obtaining the domains of desired magnetic field orientations in the memory layer, and a thermal isolation layer for isolating the memory layer from the reference layer. Laser pulses are emitted from the memory-layer side of the medium and the durations are changed in response to recording or erasing. During recording, the duration is typically decreased to 50 nanoseconds so that the memory layer is heated to above the Curie temperature, while the reference layer, which is isolated by the thermal isolation layer, is held at a relatively low temperature.
During erasing, on the other hand, duration is typically increased to 500-600 nanoseconds so as to heat the reference layer to a sufficiently high temperature. However, sufficiently long durations of laser pulses for heating the reference layer, which is isolated by the thermal insulation layer, require slow disk rotation, and hence prevent an increase in the data transfer rate. Furthermore, interposition of the thermal isolation layer is considered to make it impossible to utilize the exchange-coupling between the memory layer and the reference layer described in JA PUPA No. 62-175948.